The three element Schmidt-Cassegrain telescope is a well-known catadioptric system that can be corrected for spherical aberration, coma, astigmatism, and Petzval curvature. The invention provides a version with the same aberration correction, but with only two elements. There are just three optical surfaces that need to be made, two of which are aspheric. Because of the simplicity of this two-element design, alignment and environmental stability is better than that of more conventional systems. It is also a very compact system, with a length that is only 45% of the system focal length.
FIG. 1 shows an example of a three-element Schmidt-Cassegrain telescope and represents a design that can be corrected for spherical aberration, coma, astigmatism, and Petzval curvature. The aspheric plate 10, primary 12 and secondary 14 aspheric mirrors give the designer enough variables so that a whole family of designs with these aberration corrections can be produced. One can use this surplus of design variables to try for some additional system features, such as correction for distortion, or small obscuration, or making one of the mirrors with a spherical surface. Some of these options have been described by E. H. Linfoot in his excellent review article Recent Advances in Optics, published by Oxford University Press, 1955, page 259.
Some difficulties with the Schmidt-Cassegrain telescope are that requiring three elements complicates the mechanical design necessary to support them and protect them from distortion and position change due to temperature changes. Furthermore, although methods for replicating the aspheric Schmidt corrector plate are now known, which are fairly efficient and inexpensive, manufacture of the aspheric primary and secondary mirrors 12 and 14 is an expensive, time consuming, and, in part, hand operation.
Thus, it would be highly desirable if the number of elements in a Cassegranian system could be reduced to two, for mechanical and thermal consideration and in order to reduce the number of aspheric elements that have to be manufactured. Furthermore, it would be highly desirable if the two elements were capable of being manufactured by simple manufacturing methods such as replication. Of course a two element design would not be efficacious if it could not be provided with the same aberration corrections as the Schmidt-Cassegrain system.